William Carvalho in action for Portugal last week after his move to West Ham fell through

And Sporting are now ready to step up their case by registering a complaint with Fifa, claiming that the Hammers illegally approached the 25-year-old without their permission, according to Sky Sports.

If the case is proven, West Ham face the prospect of severe punishment, including a possible transfer ban.

The East London club had chased Carvalho for a month after boss Slaven Bilic targeted him as the man he wanted to bring into the base of his midfield.

But Sporting were reluctant to sell their Euro 2016 winner unless a buyer met their £40million valuation of him.

And with neither side willing to budge from their own valuations of the player, the window slammed shut last week without a deal being done – even though it was understood Carvalho was keen on the switch.

But since then a war of words has broken out between the two clubs, after Sporting claimed the Hammers had never even lodged a formal bid for the player.

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Sporting director Nuno Saraiva called West Ham co-owner David Sullivan a "liar" and a "parasite" after he claimed the Portuguese club had failed to give them enough time to complete a medical.

That provoked a furious reaction from the Hammers board, amid claims they were ready to sue Sporting for defamation as a result of Saraiva's statement.

David Sullivan, left, and David Gold have been accused of lying over their handling of the failed William Carvalho deal

The Hammers have insisted they have done nothing wrong and that they lodged a formal bid for the player.

Sporting are refusing to be silenced though, and club president Bruno de Carvalho stepped up the attack by labelling Sullivan and fellow owner David Gold the 'Dildo Brothers' - and challenged them to come up with proof they ever made a bid.

The Hammers promptly responded by appearing to leak the emails which they claim show they lodged a formal bid - though at £22.5m it was a long way short of the money Sporting wanted.

Now it has emerged that the Lisbon club are ready to take their own legal action by asking Fifa to investigate whether West Ham spoke to Carvalho without their permission.